Due to the remote locations of oil wells and pipelines, it is often desirable that valves at such locations be provided with means permitting remote control. Remote or automatic control is essential for automation programs and safety systems. One common method of providing such control has been to attach a fluid actuated piston operator to the stem of a rising stem gate valve. By pressurizing one side or the other of the piston, the valve can be opened or closed from a point remote from the valve. To provide fail-safe operation in the event that one of the lines carrying pressure fluid to the operator is ruptured or if for some other reason the remote control system fails, it is desirable to provide such valves with means to automatically position the valve in a predetermined safe position. In such event, it is also desirable to have the valve provided with means permitting manual operation. It is also desirable to render a manual operating system inoperative to provide a safe environment for a worker conducting repair or maintenance on a flow system controlled by a valve to which the operator might be attached.
It is also desirable to provide an actuator system that ensures against actuation of the valve by remotely located automatic sensors. If the valve is actuated while being serviced the valve and actuator assembly could be damaged or other damage could occur.
Where the valve is being employed to control high pressure fluid such fluid generally acts upon the surface area defined by the cross-sectional area of the valve stem and develops a force acting on the stem that must be overcome by the fluid motor of the actuator and by the manual override system. It would be desirable to eliminate or render inoperative the force ordinarily acting on the valve stem threads to permit easy manual operation.